1. Field of the Invention
The present invention is directed to methods and systems for sensing and compensating for process, voltage, and/or temperature (xe2x80x9cPVTxe2x80x9d) variations in integrated circuits (xe2x80x9cICsxe2x80x9d) and loads and, more particularly, for sensing and compensating for PVT variations in IC""s that are impedance matched to loads.
2. Related Art
Integrated circuits (xe2x80x9cICsxe2x80x9d) need to interface with widely varying loads, such as 40 ohm to 100 ohm transmission lines. In order to reduce voltage waveform reflections and consequent loss of signal integrity, output impedances of ICs should match load impedances.
IC characteristics and load characteristics can vary due to process (e.g., manufacturing process), voltage, and/or temperature (xe2x80x9cPVTxe2x80x9d) variations. ICs that are fabricated with narrower track widths tend to be more susceptible to PVT variations. PVT variations adversely affect circuit characteristics, such impedances and rise and fall times of waveforms.
What are needed are methods and systems for maintaining circuit and/or signal characteristics, such as impedance matching characteristics and rise and fall time characteristics, over a range of PVT variations.
The present invention is directed to methods and systems for maintaining circuit and/or signal characteristics, such as impedance matching characteristics and rise and fall time characteristics, over a range of PVT variations. The invention is useful, for example, where an IC output is coupled to a load.
In accordance with the invention, a PVT compensating circuit senses one or more circuit and/or signal characteristics at an output pad or terminal. When the one or more circuit and/or signal characteristics are affected by PVT variations in the IC and/or load, the PVT compensating circuit controls a variable output drive to maintain the one or more circuit and/or signal characteristics within a desired or predetermined range. The PVT compensating circuit is designed to compensate over a range of PVT variations.
In an embodiment, the PVT compensating circuit senses a rate of voltage change over time (i.e., dV/dt), of an output signal at the output terminal. During state transitions of the output signal, the output signal is adjusted as needed to maintain a desired, or pre-determined, rate of voltage change. As a result, the present invention enables control of output drive dependent on load.
In an embodiment, the PVT compensating circuit and the variable output drive are part of an output circuit coupled between an IC and an output terminal. The output circuit further includes an output impedance. When neither the circuit nor the load are substantially affected by PVT variations, the output impedance substantially matches the load impedance and the output signal characteristic(s) of interest are generally within a desired or predetermined range. When the IC and/or the load are affected by PVT variations, the PVT compensating circuit senses the affect and adjusts the variable output drive to make suitable corrections. In an embodiment, the output drive adjusts rise and/or fall times of an output waveform. Alternatively, or additionally, the variable output drive adjusts the output impedance of the output circuit.
In an embodiment, the output signal is initially driven by a primary output drive. Additional compensation is added as needed by the variable output drive. In an alternative embodiment, output signal is driven solely by the variable output drive circuit.
In an embodiment, the primary output drive and/or the variable output drive include impedances that, alone or in combination with other impedances, substantially match the load impedance.
In an embodiment, the output circuit is implemented on the same die or wafer as the IC. In an embodiment, the output circuit is implemented with process technology that is less susceptible to PVT variations. For example, in an embodiment, one or more portions of the output circuit are implemented using wider path widths than those used in the IC and/or load.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.